You can do a roundtrip transatlantic on QM2. Ship usually docks early in the morning and leaves again later in the day for the return journey. Would only give you a little time in New York, or you would have to stay in USA for 6 nights and catch it again next time around. Speak to Travel Agent or direct with Cunard.

There are about 3 Atlantic crossings by Cunard which tie in the eastbound-&-westbound voyages. The brochure identifies them. In particular the voyages round the Fourth of July cater for this, as QM2 is often chartered for the days round 4 July so it works well for them. It is a simply wonderful way of seeing NY - 25-hour days on the westbound voyage, balanced by 23-hour days eastbound, so you don't have any time-lag at all.